The process for controlling pests, particularly ectoparasites in sheep and cattle by dipping has been in use for many years. Dipping is an inexpensive method for the control of a wide range of pests, including ectoparasites of the classes Insecta and Arachnida, and particularly including Lucilia cuprina and Lucilia sericata responsible for blowfly strike of sheep; Bovicola ovis (Sheep body (biting) louse) infestation of sheep; Psorergates ovis (itchmite) infestation of sheep; Psoroptes ovis (sheep scab mite) infestation of sheep; Melophagus ovinus (Sheep ked) infestation of sheep; Damalinia caprae (goat body (biting) louse) infestation of goats; Boophilus microplus (Cattle tick), Haemaphysalis spp. (Bush ticks), Rhipicephalus evertsi and Aponomna spp (red legged ticks), Rhipicephalus appendiculatus (brown ear tick) Hyalomma spp (Bont-legged tick) and Ambylomma spp. (Bont tick) infestation of bovid animals; Damalinia bovis (biting louse), Haematopinus eurysternus (short-nosed sucking louse), Haematopinus tuberculatus (buffalo louse), Haematopinus quadripertusus (tail switch louse), Linognathus vituli (long-nosed sucking louse) and Solenopotes capillatus (little blue sucking louse) infestations of cattle; Chorioptes bovis (Scrotal mange mite) infestation of cattle and sheep; Haematobia irritans exigua (Buffalo fly, Horn fly) annoyance of bovids; Hypoderma bovis (Warble fly) infestation of bovids; Glossina sp. (Tsetse fly) carrying sleeping sickness, a disease of economic importance principally in cattle; Chrysomya spp. (screw worm flies) of many livestock species; Musca spp. and Stomoxys calcitrans (Stable fly) which cause annoyance of mainly housed or intensively managed livestock.
Two types of dipping methods are commonly used, namely plunge dipping and shower dipping (which is also referred to as spray dipping). In plunge dipping the animals are fully immersed in dipping liquid contained in the dip or sump (vessel holding liquid). In shower or spray dipping a pump draws liquid from a sump (vessel holding liquid) and delivers the liquid via pipes to nozzles which spray the animals. Excess liquid returns to the sump via floor drains.
In the plunge dipping method, animals leaving the dip retain some of the dipping liquid in or on their exterior. This liquid may be carried away from the dip, causing a loss of dipping liquid from the dip. This loss of liquid is generally proportional to the number of animals which have passed through the dip.
In the shower dipping method, batches of animals (in the case of sheep, typically 25-70 animals/batch) are herded into a dipping station and are sprayed with the dipping liquid for a time period specified to adequately wet the animals. The liquid level in the sump of the shower dipping station initially drops rapidly as liquid is directed to the shower nozzles, however drains are generally provided to return liquid from the floor of the dipping station to allow its recycling. Some of the dipping liquid is retained in or on the surface of the animals and as a result the total dipping liquid is continually reduced. When the showering operation is completed, the animals leave the dipping station and the dipping liquid returns via the floor drains, resulting in a rapid rise in the liquid level in the sump. For smaller sumps, where the volume of dipping liquid in the return lines is a significant fraction of the total sump volume, the difference between sump level during and after dipping is proportionally greater. As referred to above, however there is also a net loss of dipping liquid from the sump between beginning and end of each batch treatment, and this net loss is directly proportional to the number of animals which have passed through the station.
The quantity of dipping liquid which is lost during plunge and spray dipping must be replaced when significant numbers of animals are to be treated. Replacement methods can be characterised either as intermittent or continuous. Intermittent replacement methods are characterised by replacement of dipping liquid in the sump only at discrete time points (between these discrete time points no replacement of dipping liquid occurs). In practice, intermittent replacement methods can involve:
periodic topping up of liquid in the sump (for both plunge and shower dipping stations). This is undertaken when some fixed portion, preferably no greater than 25%, of the sump volume has been expended, or PA1 batch-by-batch topping up of liquid in the sump (for both plunge and shower dipping stations). This is undertaken after each batch of animals has left the dipping station. PA1 establishing a predetermined level of a dipping liquid containing a first concentration of the pesticide (C.sub.IC) in a vessel said concentration providing safe and effective pesticidal treatment; PA1 dipping the animals in the dipping liquid; and PA1 maintaining the predetermined level of the dipping liquid in the vessel by addition thereto of a replenishment composition containing a second concentration of the pesticide PA1 comparing the graph determined in step (i) with the array of calculated stripping curves to determine the stripping factor corresponding to the closest fitting calculated stripping curve.
Continuous replacement methods are those wherein replacement liquid is available for progressive addition to the sump at all or most times throughout the dipping process. Replacement is usually achieved by the transfer of dipping liquid from a holding tank or vessel to the sump. It is important to note that in continuous replacement methods, the rate of addition of dipping liquid from the holding tank to the sump need not be steady, but may be adjusted in response to rate of loss of dipping liquid from the sump.
Hereinafter, the practice or method of periodic topping up or replacement of dipping liquid in the sumps of plunge or shower dipping stations will be denoted as periodic replenishment. Similarly, batch-by-batch topping up or replacement will be denoted batch replenishment and continuous topping up or replacement will be denoted continuous replenishment.
In replenishment methods, the quantity of dipping liquid in the sump of the dipping station is regulated either by:
a) maintaining its height at some predetermined level, or PA0 b) maintaining its height within a range which is fixed by predetermined lower and upper levels. PA0 (a) the nature of the active ingredient; PA0 (b) the nature of the formulation comprising the active ingredient, and in particular the nature of the solvent in the said formulation, if the active ingredient is maintained in the dipwash as an emulsion; PA0 (c) the dipping method (plunge or shower dip); and PA0 (d) the sump volume of the dipping station, or some other factor(s) highly correlated with sump volume. These related factors presumably could include features of dip operation that influence degree of exposure of the dipping liquid (active ingredient) to the stripping effect of the animal surfaces (e.g., length of the bath, swim distance and/or time in plunge dips, showering time/sump turnover in shower dips). PA0 (i) dipping a number of animals in a dipping liquid of initial concentration C.sub.IC contained in the vessel and maintaining the level of liquid in the vessel during dipping by replenishment of the liquid removed with a dipping liquid of the first concentration equal to C.sub.IC and preparing a graph of concentration of pesticide against numbers of animals. PA0 (ii) generating an array of calculated stripping curves showing the change in concentration of pesticide during dipping for a range of SF values using the formula ##EQU1## wherein V.sub.s is the volume of dipping liquid
For periodic replenishment plunge dips, the level of dipping liquid in the sump of the dipping station gradually declines from its maximum to its minimum predetermined level (eg. 75% of maximum). For periodic replenishment of shower dips, the level of dipping liquid in the sump of the dipping station when measured at immediately before the start of each batch gradually declines from its maximum to its minimum predetermined level (eg. 75% of maximum). For batch replenishment plunge dips, the level of dipping liquid in the sump of the dipping station gradually declines as animals of the batch pass through the dipping station. For batch replenished shower dips, the level of dipping liquid in the sump of the dipping station when measured at the conclusion of each batch is somewhat less than the level of dipping liquid in the sump when measured immediately before the start of each spray batch. For continuously replenished plunge dips, the level of dipping liquid in the sump remains steady at a single predetermined level (minor variations of less than 10% may occur in practice). For continuous replenishment shower dips, the level of dipping liquid in the sump immediately before the start of spraying of a given batch is the same as the level in the sump immediately before the start of spraying of the next batch (again minor variations of less than 10% may occur in practice).
A significant problem in the dipping process is stripping. Stripping is a process whereby animals preferentially remove the active (pesticidal) ingredient in the dipping liquid relative to the volume of dipping fluid removed. Stripping may lead to a number of undesirable consequences.
As a result of stripping the concentration of active ingredient in the sump decreases as increasing numbers of animals pass through the sump.
The latter portion of animals passing through the dipping station commonly obtain an undesirably low dose of active ingredient, leading to insufficient protection against pest species. The retention of pest activity on even a few dipped animals can lead to the rapid reinfestation of the entire group of animals increasing the risk to the health of the entire animal population. Additionally, exposure of pests to sublethal doses of pesticide can foster development of pesticide resistance.
The reduced dose received by latter animals can be offset by increasing the concentration of active ingredient in the initial dip, however this has the undesirable consequence that the first animals through the dipping station may obtain an undesirably high dose, leading to problems of pesticide contamination of animal products such as wool and meat. In wool processing, the accumulation of pesticide in lanolin may lead to effluent disposal problems following scouring of the fleece. Toxic effects for animals exposed to high pesticide doses are also possible.
As a consequence of removal of dipping liquid by animals passing through the dipping station and preferential removal of active ingredient in the stripping process, the problem of applying a uniform dose to all animals passing through the dipping station is complex.
British Patent 2,186,474 discloses a process in which a special dosing device dispenses concentrated active ingredient in proportion to the number of animals passing through the dipping sump (countering the effect of stripping) and the fluid level in the sump of the dipping station is also maintained by addition of diluent which is substantially water. An important component of the method of Stratford et al is the use of a sheep counting device to enable the correct dose of concentrated active ingredient to be applied to the sump. The method of Stratford et al is theoretically capable of providing a uniform dose of pesticide to all animals passing through the dipping station however the method has not been accepted by farmers. This is presumably because of the cost, complexity and inconvenience of installing, maintaining and calibrating the device for dispensing active ingredient in response to animal number. The requirement for calibration of this device is particularly inconvenient since the same active ingredient may strip at different rates in different use contexts.
Another approach to overcoming the problem of stripping of active ingredient in a dipping operation is to change the formulation of active ingredient. It has been noted that microencapsulation of the active ingredient overcomes the stripping problem (Ciba Geigy patent AU-B-80034/87) and it has also been found that changing the solvent carrier in a formulation of active ingredient can reduce the propensity of the pesticide on to undergo stripping. Reformulation to avoid stripping has the disadvantage of either high formulation cost (associated with microencapsulation) or insufficient reduction of stripping factor (associated with solvent variations). The pesticidal efficacy of microencapsulated formulations may also be reduced, particularly when rapid initial insect knock-down is desirable or when rapid diffusion of active ingredient through the wool grease is required.